Many and various layouts have been adopted for vehicles and their drivetrains. The principal distinctions lie in the selection of which wheels are driven, and the location of the engine and gearbox that drive those wheels. The most common layout is front-engined, in which the engine is located forward of the passenger cell. However, for reasons of packaging and weight distribution it is also known to adopt a mid-engined layout or a rear-engined layout. A rear-engined layout, such as that adopted in the VW Beetle and the Porsche 911, places the engine over or behind the rear axle. A mid-engined layout places the engine between the rear axle and the passenger cell, and is often employed in sports cars and supercars such as the Lotus Elise, Porsche Boxster and Carrera GT, the Ferrari 430, and the Lamborghini Gallardo (for example).
The engine of any vehicle must be cooled, wherever it is located. In front-engined vehicles, this is usually achieved by placing a radiator behind a grille on the front face of the vehicle. As the vehicle moves forward, cold air flows through the grille and through the radiator. Meanwhile, a heat transfer fluid is pumped around a circuit that includes the engine and the radiator. The fluid is therefore cooled by the air flowing through the radiator, and (in turn) cools the engine through which it flows.
Rear- and mid-engined vehicles typically use essentially the same arrangement, extending the conduits to and from the radiator as necessary in order to reach the engine. Some early rear-engined vehicles (such as the early Beetle and 911 variants) dispensed entirely with forced cooling arrangements, and instead ducted air flowing over the roof of the vehicle into the engine bay and over cooling fins built into the engine block and/or cylinders, thereby air-cooling the engine.
The heat transfer fluid is usually water-based (i.e. water containing various additives, such as to inhibit freezing and corrosion). Other fluids may be used however, such as the Porsche 911 (type 993) which employed the lubricating oil as the heat transfer fluid.